The renaturation/denaturation of DNA oligonucleotides is characterized in the contextof expanded ensemble (EXE) and transition path sampling (TPS) simulations.Free energy profiles have been determined from EXE for DNA sequences ofvarying composition, chain length, and ionic strength. TPS simulations within aLangevin dynamics formalism have been carried out to obtain further informationof the transition state for renaturation. Simulation results reveal that freeenergy profiles are strikingly similar for the various DNA sequences consideredin this work. Taking intact double-stranded DNA to have an extent of reactionξ = 1.0, the maximum of the free energy profile appears atξ≈0.15, correspondingto ∼2 base pairs. In terms of chain length, the free energy barrier of longer oligonucleotides (30versus 15 base pairs) is higher and slightly narrower, due to increased sharpness associatedwith the transition. Low ionic strength tends to decrease free energy barriers, wherebyincreasing strand rigidity facilitates reassociation. Two mechanisms for DNAreassociation emerge from our analysis of the transition state ensemble. Repetitivesequences tend to reassociate through a non-specific pathway involving molecularslithering. In contrast, random sequences associate through a more restrictivepathway involving the formation of specific contacts, which then leads to overallmolecular zippering. In both random and repetitive sequences, the distribution ofcontacts suggests that nucleation is favored for sites located within the middleregion of the chain. The prevalent extent of reaction for the transition state isξ≈0.25, and the critical size of the nucleus as obtained from our analysis involves∼4 base pairs.